The surface wave

At the frequencies below around 10 MHz, the earth’s surface conducts alternating current well. Because of this, vertically polarized EM waves follow surface for hundreds or even thousands of miles, with earth helping to conduct E flux. The lower frequency, lower the ground loss and farther the waves travel by surface-wave propagation. The horizontally polarized waves do not travel in this mode, as horizontal E flux is shorted out by earth.

Above around 10 MHz, the earth becomes lossy, and surface-wave propagation is not useful for over a few miles. Significant surface-wave communications are done, majorly at very low and medium frequencies